Glide path control for aircraft



June 3, 1952 D, n-z

GLIDE PATH CONTROL FOR AIRCRAFT 2 SHEETSSHEET 2 Filed March 28, 1946 m m m m BY 5617129] Z Z Z a@. ll

ATTORNEY Pat ented June 3, 1952 Daniel Blitz, Princeton, N. .L, assignor to Radio Corporationof America, a corporation ofDelaware Application March 28, 1946, Serial No. 657,688

9 Claims. 1

My invention relates to radar control systems and particularly to methods of and means for controlling the glide path of an aircraft as it approaches an airport for landing.

An object of the invention is to prcvidean improved method of and means for controlling the approach glide path of an aircraft over land, particularly over land that is irregular or hilly in contour. 7

Another object of the invention is to provide an improved system for causing an aircraft to follow a predetermined glide path in approaching an airport for a landing.

In certain copending applications there have been described systems for causing an aircraitto follow automatically a glide path over water for approaching a surface ship. For example, a drone loaded with explosives may he flown into an enemy ship in this Way either in daylight or after dark or in a heavy fog. As an example of such a copending application, reference is made to application Serial No. 542,989, new Patent No. 2,454,009, issued November 16, 1948, filed June 30, 1944, in the nameof Royden C. Sanders, J r.,-and entitled RadarContro'l system.

The present application describes a system for causing an aircraft to follow a predetermined glide path over land of irregular contour as, for example, where the approach to an airport is over hilly terrain. According to the present invention the system includes a cam that has a contour corresponding to the profile of the terrain over which an aircraft is to fly in approaching an airport. Means is provided to transmit a control signal from a ground station to the aircraft control equipment which includes a radio altimeter. This control signal has a characteristic, such as frequency, that is a function of the contour of said cam. The control signal causes the aircraft control equipment to hold the aircraft on a smooth predetermined glide path regardless of the fact that the altimeter output signal is being changed by the hilly terrain. The control equipment on the aircraft may in large part he similar to that described in the above-mentioned Sanders application.

[The invention will be better understood from the following description taken in connection with the accompanying drawingin which Figure 1 is a view showing, by way of example, an approach glide path for an aircraft and-showing also the contour in elevation of the terrain over which the aircraft makes the approach,

Figure 2 is a block and circuit diagram of a ground station embodying the invention,

2 FigureZa is-a view showing a cam that may :be used in the system shown in Fig. 2 where the terrain is level,

Figure 2b is a view showing another embodi ment of a portion of the apparatus shown in Fig. 2, and

Figure 3 is a circuitdiagram showing the=aircraft control equipment according .to one embodiment of the invention.

Referringto Figs. 1 and 2, a control tower :4 containing the apparatus of fig. :2 is located rat the airport and at one side of a runway. This ground equipment comprises va frequency modulated or PM radar system that includes an transmitter 2 and a beat frequency detector 3; it further comprises a broadcast transmitter t that transmits an audio frequency signal, theatrequency of which is determined .by a :camfi.

Referring more specifically to the ground radar station, it is similar to the well known FM'altimeter but in the present instance it measures the distance to the approaching aircraft, i. e., the distance from the tower l to the aircraft .6 shown in Fig. l. The transmitter v2 is cyclically 119- quency modulated by a triangular wave supplied from a generator '1. Some -;of the frequency modulated signal is supplied from the transmitter 2 to the detectoritsothat it beats withzthe signal reflected from an object, such as the aircraft 6, to produce an audio frequency signal which has a frequency that-is 'a-function .of the distance to said object.

The audio frequency signal issupplied from the detector 3 through an amplifier 8 to a frequency measuring circuit which comprises a limiter tube 9 and a counter circuit I0. Thezcircuit 1.0., which is of a well known type, comprises .a-comparatively small capacitor vl I :and adiode 12 through which a pulse of energy is supplied to a comparatively large capacitor I 3 each time a negative half cycle occurs in the square wave supplied :from the limiter '9. The capacitor H, sometimes referred to as the bucket capacitor, is discharged through'the diode LIA at the end of each negative half cycle.

A leakage path and a bias cOnnectiQnare-providedfrom the anode end-of capacitor 13 through a resistor 121, a conductor I28, a potentiometer arm I29 and a potentiometer resistor 13-! which is connected to a voltage source not shown. The voltage across the resistor l 21 reaches :an equilibrium value in .operation :and is a measure of the distance to the approachingaircraft. Thepotentiometer shaft I32 carrying the arm I29 is caused to assume an angular position that is a function of said distance, this being accomplished by means of a follow-up circuit that comprises an amplifier tube I33, a relay I34 and a reversible motor I36.

The grid of the tube I33 is connected to the anode end of capacitor I3 so that the anode current of the tube I33 changes with any change in the voltage across the capacitor I3. The anode current operates the marginal relay I34 which has an armature I3'I that moves into contact with either its upper contact point or its lower contact point if the anode current departs from a predetermined value. This causes the motor I3t to run in one direction or the other to drive the potentiometer arm I23 in a corresponding direction. As a result, the bias voltage applied through the lead I28 and the resistor I2'I is changed. in the direction tending to stop the motor I33. As the aircraft 6 approaches the runway, the angular position of potentiometer shaft I32 changes and is always a function of thedistance to said aircraft.

The cam is coupled to the shaft I32 so that. its angular position is also a function of the distance to the aircraft 6. Thus, the cam follower I always rides on a portion of the cam where the cam contour corresponds to the contour of the terrain underneath the approaching aircraft. The shaft I42 of the cam follower I4I is coupled to the arm I43 of a potentiometer I44 so that the position of the arm I43 is determined by the cam 5.

Referring again to the broadcast transmitter 4, it is modulated by the output of an audio frequency oscillator I46 which is supplied to a suitable transmitter modulator I4'I. The frequency of the oscillator I46, which may be any one of many well known types, is determined by the position of the potentiometer arm I43 and, therefore, by the contour of the cam 5. Thusthe audio control signal that is transmitted from the transmitter 4 has a frequency that is a function of the distance from the tower I to the aircraft 6 and also a function of the contour of the terrain.

. It may be noted that if the approach to the runway is over level ground, the shape of the cam will resemble that shown in Fig. 2a, i. e., the cam contour will change uniformly so that the control signal will decrease uniformly in frequency as the distance from the tower I to the aircraft 6 decreases. In the example illustrated in Figs. 1 and 2, the cam contour causes a still further decrease in control signal frequency when the aircraft is over a hill as shown in Fig. l, and thus the ratio of control signal frequency to altimeter signal frequency is constant so long as the aircraft is on the proper glide path.

Instead of shaping the cam so that its contour is a function of both distance and terrain elevation, it may be preferable to employ the arrangement of Fig. 2b where the cam contour is a function of the terrain elevation only. In this case the potentiometer arm I43 is driven by the distance shaft I32 through a differential unit I50. The shaft I32 is coupled directly to one input shaft I5I of the unit I50 and is coupled by Way of a cam 5a and a cam follower I4Ia to the other input shaft I52. In this example the cam 5a will be circular in contour if the terrain under the approach path is level.

It may be noted that if other than a straight line glide path is desired, this may beobtained by selectinga suitable contour for the cam. For

instance, in the above-mentioned example where the cam 5a is circular for a straight line glide path, it may be given a different contour, such as a spiral contour, for glide path that becomes less steep as the aircraft approaches the landing strip.

The manner in whichthe audio frequency control signal is utilized at the approaching aircraft 6 for controlling the glide path will now be described with reference to Fig. 3.

The radio signal which is modulated by the audio frequency control signal is picked up by an antenna I1 and supplied to a receiver I6 and a detector I8 where it is demodulated. The resulting audio frequency signal is passed through an amplifier I8 to an amplitude limiter 20 and the resulting square wave signal is applied to a frequency counter I9. The amplifier I3 preferably is provided with an automatic volume or gain control circuit I811.

The counter I9 comprises a pair of oppositely connected diodes in a common envelope 2| to which the square wave from the limiter 28 is applied through a capacitor 22 of comparatively small capacity. A storage capacitor 23 of comparatively large capacity has a charge supplied to it through the cathode 24 and anode 26 of one diode section upon the occurrence of each positive half cycle of the square wave. Since the capacitor 22 is small enough to reach full charge during the first part of a square wave half cycle, the storage capacitor 23 is charged up a fixed additional amount each time a positive half cycle occurs whereby the voltage thereacross is proportional to the frequency of the audio frequency control signal. This is assuming for the moment that the charge on capacitor 23 is not being affected by the altimeter counter I9 circuit described hereinafter. The voltage appearing across the capacitor 23 is applied to the grid of a vacuum tube 3| through a tube protective resistor 32.

The diode section which comprises a cathode 21 and an anode 28 is provided to discharge the capacitor 22 at the end of each positive half cycle. The anode 28 is connected through a lead 30 to an intermediate point on a cathode resistor 29 of the vacuum tube 3I (rather than directly to the cathode of tube 3 I) to prevent current flow through the diode 21, 28 due to contact potential and to maintain as nearly as possible the charge delivered each cycle by capacitor 22 a constant value independent of frequency, thus making the output of the counter very nearly linear with respect to frequency.

A radio altimeter is provided which includes the counter I9, previously mentioned, which is connected so that its diodes and those of the counter I9 supply current to the capacitor 23 in opposing or differential relation.

The altimeter comprises a frequency-modulated transmitter 33 that radiates the signal downwardly from an antenna 33. The transmitter may be frequency modulated by a modulating oscillator 34 which supplies a triangular wave modulating signal, for example. The band width of the frequency modulation sweep may be adjusted by means of a variable tap 34'. I The reflected signal is received by an antenna 35 and supplied to a detector 32 where it beats with the frequency modulated signal supplied directly from the transmitter 33 to produce an audio signal having a beat frequency that corresponds to the altitude of the aircraft 6.

The beat frequency signal is supplied through am ifie 7 a dsthroush an nr n n elhn te na 2 t th i equeno coun e .9- Th .e nte 9 i o th m g n ra ty e a th counte 19 u i a ne c u -wh e s the counter I 9 is a positivecounter. The parts in eonit 9 .eorres ondine t thos i th c unter i9 are indic t d b e me ie e enee nume l w thor ime Inerk e de T e sto a ca a to i eommon-t h -W d oc se ons 4.1 .e-ns .2 th ;first on su p yin cu r n to sea ona 1 du in th Po e ha cir e o the au f e u nc c o si na to mak i upper terminal more positive, and-theother section 24, 26' supplying current to capacitor 23 r n th n g v .he ycl of th a t u signal to make its upper terminal less positive. Thu th p t nt a etsaid u e te m n i t e i ereno i th o ut of t tw c u ers. and heio n t o the t o c ter a p p tional to the control signal-frequency and ,to the altimeter beat frequency, respectively. So long as the aircraft is on the proper glide path, the difference of the two counter outputs is zero. It will be noted that the cathode 2 is connected .to the cathode of the cathode-follower tube -3!, instead of to ground, whereby the negative counter I? also is made substantially linear. Bias for follow-up control is applied to the counters l9 and 19' through a resistor 38 which is connected through a conductor 39 to a follow-uptap All on o a voltage divider resistor 5|. The resistor ,4! is connected across a resistor 62 which isone section of a voltage divider comprising resistors 43, s2 and a The cathode follower tube 3| is coupled to a vacuum tube 45 which has its control grid held at a fixed bias having a value determined by the setting of a variable tap it on the resistor 42. The plate circuit of the tube 45 includes a relay coil 41 for actuating an armature 48. The relay 47, 48 is operated around the point where the plate current of the tube 3| equals the plate current of the tube 45, on one side of this point the tube 3| going to plate current cut-off and on the other side of this point the tube %5 going to plate current cut-off due to current flow through the cathode resistor 29.

Thus, the relay armature 43 connects a D. C. source 49 through one of the conductors 50 and 5| to a reversible motor 52, referred to as the pitch motor, with the correct polarity for either forward or reverse operation, depending upon whether the differential output of the counters l9 and I9 is above or below a predetermined value. The D. C, operating voltage for the limiter and counter tubes is taken from a common source to avoid any unbalance due to changes in the operating voltage amplitude. The pitch motor 52 is mechanically coupled to the follow-up tap 40 whereby the counter bias is changed when the motor 52 rotates, the bias change being in the direction to stop the motor rotation.

it h s be sho n o t e m to .2 is a d t r ta e in n re ti n o the her (m v n the i lew nn en 4 t it) res s to a partare from h des red at o f ntro s nal frosne ev to altimeter ea f equ n caus b he aircraftgetting either above or below the desired s de ath Th ro a io of th to funoe tions primarily to control the elevator surfaces of the aircraft for bringing it back to the desired glide path. In the example illustrated, the motor 52 is tied in with an automatic pilot mechanism known construction that also controls the elerot r sorfa es n i ll n e d so ib d The automatic-pi ot ino ude n ionsi udinn ee ,t ud con ol yroscope 5. p ovided with agile.- he r n v carry. g two .-.conductin -sectors +53 and 5. separated l y a small insu ating secto -.61. .hl ontae 'fi en agin either-.the;sector;6 l .orione o h conduc in -.seotors 58 shots). .is-on-the end o a le er .63 hat is a tached to a -,uivota11v m n p rtin m mber .64 so that the-non- .tee .5 il be u ed in an.-arcuaterath.abo the sector :58. 5 an .6 The seotorsiia and 5:9 .are connected to ,two terminal 9 arer rsihl o ow-up motor 65 w i e th oQntaQt-BZ is cone t dth ouehthelever eeand-throu hadirectcurrent source 61 -;to a third terminalof theme tor v6 6. The shaft of the motor 66 is mechanically coupled through a linka e 6:1 totbeelevatgr surfaces (notshown) oftheairplane.

he ont ol st ck 68 of theair n :;i. connected at a pivot :69 to the;control linkage and through an arm H to a cable-I2. Thecable i2 is guided over a plurality of pulleys 'lfi :and a pulley 14 and connected to the .supportingarm 61} carrying the contact 62. Aspr-ing 3161s provided to maintain the cable 12 under tension. The pulley M is supported at the end of .a'glever T! secured to the shaft .of the reversible pitch motor 52.

Neglecting temporarily the effect of -.operati-ng the motorfiz, the operation of the automatic pilot system when adjusted for level flight is as fol-.- lows: The gyroscope 53 tends to maintain econstant attitude, with its rotor in a plane parallel to the surface of the earth. The movable contact 62 normally engages the insulating sector 51. Any deviation of the airplane from level flight will move the contact 62 with respect to the ring 51, and into contact with sector 58 andj.59.

Thus the motor 65 will be energized so as to run in the proper direction to adjust the elevator control surfaces to cause the airplane to resume its attitude for level flight.

When the motor 66 is operating to adjust-the elevator surfaces, it also moves the cable H, thereby rotating the contact 62 with respect to the longitudinal axis of the craft. When the contact 62 reaches the insulated sector 6|, the motor 66 is deenergized. During this time the elevator surfaces have been bringing the aircraft back toward the position of level flight. As the airplane continues toward its normal attitude, the contact 62, which has been displaced ahead of the gyroscope, passes the insulatedsector and engages the opposite conducting sector, causing the motor 66 to run in the reverse direction. This returns the control surfaces toward the position for a level flight. Thus, the applied control is removed as the airplane is returning to its nor.- mal attitude, so that the control surface will be back in its neutral or central position when the disturbance has been corrected. Briefly, a follow-up action has been applied to control the aircrafts attitude as a function of the gyro control.

The operation of the complete system while holding the aircraft on the desired glide path will now be described, assuming a method of operation where the automatic pilot adjustment is the one previously described which holds theair craft in level flight in the absence of a control action from the differential counter circuit. the aircraft approaches the airport it automatically either increases or decreases altitudeto, get on the glide path and remains on this path, the e n r ope on. b i g as ollows:

The relay ar ature 41 is moved to either lower or upper position depending on whether the differential output of counters l9 and I9 is greater or less than a certain predetermined value, thus energizing the pitch motor 52 to move the pulley l4, displacing the contact 62 from the level flight position, and causing the attitude of the airplane to change in the direction for either increasing or decreasing descent.

At the same time, the pitch motor 52 also moves the follow-up tap .40 along the resistor 4!, thus changing the counter bias voltage applied through resistor 38 to the counters l9 and I9 in the direction to reverse the position of the relay armature 48. The lever 1'1 and the follow-up tap 4B are normally centered for level flight in the method of operation being described. Assume that as the airplane moves toward the airport, its rate of descent is too gradual so that its flight path is above the desired glide path. Since the ratio of altimeter output frequency to control signal frequency is too high, the relay 41, 48 is actuated to start the motor 52 and thus change the position of the pulley M. This moves the .contact 62 with respect to the gimbal ring 51,

operating the motor 66 to change the flight attitude so as to bring the airplane to the correct glide path. Motion of the pitch motor 52 also moves the follow-up tap 40, changing the counter bias voltage in the sense to increase the current in the relay coil 47 whereby as the airplane approaches the desired glide path, the relay 4?, 48 is operated to reverse the motor 52, returning the follow-up tap 4B and the contact 82 to their normal positions for the correct glide path. In the example just described, mechanical control ratios between the motor 52, the pulley '14 and the follow-up tap 4B are such that the contact 52 is centered when the craft is in level flight.

Instead of adjusting the system so that the automatic pilot gyroscope 55 tends to hold the aircraft in level flight in the absence of the differential counter control, it may be preferred to adjust or bias the gyroscope so that the automatic pilot itself holds the aircraft approximately on the correct glide path. In this method of operation, the radio control system has greater operating range in holding the aircraft exactly on the desired glide path; it now has only to correct for the amount that the gyroscope 56 fails to hold the aircraft on the desired glide path. The following procedure may be practiced when this method of operation is employed:

As soon as it is desired that the aircraft shall start on the glide path, the bias of the gyroscope 56 is changed to the glide path adjustment and, at the same time, the radio differential counter control is switched in as by closing a switch 80 in the power supply circuit for the pitch motor 52.

It will be understood that when the aircraft controls are switched over to the differential counter glide-path control, the aircraft will immediately seek the correct glide path and will either climb or descend to reach this path unless it happens to be on the correct glide path at the time.

I claim as my invention.

1. An aircraft control system comprising a radar system located at an airport for determining the distance to an approaching aircraft,

means comprising a radio system at said airport for transmitting to said aircraft a control signal having a frequency that is a function of said distance, and a radio control system on said aircraft whereby it may be held on a predetermined glide path, said aircraft borne control system comprising a radio altimeter that supplies an output signal having a frequency that is a function of altitude and comprising a radio receiver for receiving said control signal, and further comprising means responsive to a change in the ratio of the frequency of said altimeter output signal to the frequency of said control signal.

2. An aircraft control system comprising a radar system located at an airport for determining the distance to an approaching aircraft, means comprising a radio system at said airport for transmitting to said aircraft a control signal having a characteristic that is a function of said distance, and a radio control system on said aircraft for holding said aircraft automatically on a predetermined glide path, said aircraft borne control system comprising a radio altimeter that supplies an output signal that is a function of altitude and further comprising a radio receiver for receiving said control signal, and means for controlling the altitude of said aircraft as a function of said altimeter output signal and said control signal.

3. An aircraft control system comprising a radar system located at an airport for determining the distance to an approaching aircraft, means comprising a radio system at said airport for transmitting to said aircraft a control signal having a characteristic that is a function of said distance and also a function of the contour of the land over which said aircraft is approaching, and a radio control system on said aircraft for holding said aircraft automatically on a predetermined glide path, said aircraft borne control system comprising a radio altimeter that supplies an output signal that is a function of altitude and further comprising a radio receiver for receiving said control signal, and means for controlling the altitude of said aircraft as a function of said altimeter output signal and said control signal.

a. An aircraft control system comprising a radar system located at an airport for determining the distance to an approaching aircraft, means comprising a radio system at said airport for transmitting to said aircraft a control signal having a frequency that is a function of said distance and also a function of the contour of the land over which said aircraft is approaching, and a radio control system on said aircraft for holding said aircraft automatically on a predetermined glide path, said aircraft borne control system comprising a radio altimeter that supplies an output signal having a frequency that is a function of altitude and further comprising a radio receiver for receiving said control signal, and means for changing the altitude of said aircraft in response to a change in the ratio of the frequency of said altimeter output signal and the frequency of said control signal.

5. In an aircraft control system, a radar system located at an airport for determining the distance to an approaching aircraft, said radar system including follow-up means for causing a shaft to assume an angular position that is a function of said distance, a cam that is coupled to said shaft for rotation therewith, said cam having a contour that is a function of said distance, a radio transmitter for transmitting a control signal to said aircraft, and means for modulating said transmitter in accordance with the contour of said cam as it is rotated by said shaft.

6. In an aircraft control system, a radar system located at an airport for determining the distance to an approaching aircraft, said radar system including follow-up means for causing a shaft to assume an angular position that is a function of said distance, a cam that is coupled to said shaft for rotation therewith, said cam having a contour that is a function of both said distance and the contour of the terrain over which said aircraft is approaching, a radio transmitter for transmitting a control signal to said aircraft, and means for modulating said transmitter in accordance with the contour of said cam as it is rotated by said shaft.

7. In an aircraft control system, a radar system located at an airport for determining the distance to an approaching aircraft, said radar system including follow-up means for causing a distance shaft to assume an angular position that is a function of said distance, a cam having a contour that is a function of the contour of the terrain over which said aircraft is approaching, a differential unit having two input shafts and an output shaft, means for coupling said distance shaft directly to one of said input shafts, means for coupling said distance shaft to the other of said input shafts through said cam whereby said output shaft is rotated as a function of said distance and said cam contour, a radio transmitter for transmitting a control signal to said aircraft, and means for modulating said transmitter in accordance with the rotation of said output shaft.

8. Air borne equipment for causing an aircraft to follow a predetermined glide path to a landing strip, said equipment comprising a radio altimeter that supplies an output signal having a characteristic that is a function of altitude, a radio receiver for receiving from a ground station a control signal having a characteristic that is a function of the distance to said landing strip and also a function of the contour of the terrain 10 under said aircraft, and means for controlling the altitude of said aircraft as a function of both the said altimeter output signal and said control signal.

9. Air borne equipment for causing an aircraft to follow a predetermined glide path to a landing strip, said equipment comprising a radio altimeter that supplies an output signal having a frequency that is a function of altitude, a radio receiver for receiving from a ground station a control signal having a frequency that is a function of the distance to said landing strip and also a function of the contour of the terrain under said aircraft, and means for controlling the altitude of said aircraft in accordance with the ratio of the frequency of said altimeter output signal and the frequency of said control signal.

DANIEL BLITZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,187,097 Pope Jan. 16, 1940 2,252,083 Luck Aug. 12, 1941 2,322,448 Holmes June 22, 1943 2,372,620 Williams Mar. 27, 1945 2,412,003 Neufeld Dec. 3, 1946 2,412,632 Sanders Dec. 17, 1946 2,421,106 Wight May 27, 1947 2,423,336 Moseley July 1, 1947 2,433,782 Murdock Dec. 30, 1947 2,436,846 Williams Mar. 2, 1948 2,454,009 Sanders Nov. 16, 1948 2,459,482 Bond Jan. 18, 1949 2,466,534 Cole Apr. 5, 1949 

